In recent years, secondary batteries such as lithium secondary batteries (and typically, lithium ion batteries) or nickel hydrogen batteries have become increasingly important as power supplies for vehicles using electricity for their driving power source and as power supplies installed in electric products such as personal computers and portable terminals. Lithium secondary batteries in particular are expected to be used as high-output, vehicle-mounted power supplies since they are lightweight and allow the obtaining of high energy density, and their demand is expected to increase.
In a typical configuration of this type of lithium secondary battery, electrodes (positive electrode and negative electrode) are provided that employ a configuration in which electrode active material layers (and more specifically, a positive electrode material layer and a negative electrode active material layer), mainly composed of an electrode active material capable of reversibly occluding and releasing lithium ions, is formed on electrode current conductors. In addition, these electrode active material layers are formed by coating a composition, prepared in the form of a paste or slurry by dispersing an electrode active material in a suitable solvent, onto an electrode current conductor followed by drying and compressing with a roller and the like.
Examples of the prior art relating to this type of electrode of a lithium secondary battery are disclosed in Patent Documents 1 to 3. Patent Document 1 discloses a technology for preventing loss of an electrode active material by providing a porous protective film on the surface of either of a positive electrode active material and negative electrode active material layer. In addition, Patent Document 2 discloses a technology for controlling the pore structure of a porous film by setting an average particle diameter and the like of α-alumina particles of a porous film composed of an inorganic oxide filler (α-alumina particles) and a binder. Moreover, Patent Document 3 discloses a technology for improving durability in a lithium secondary battery having a ceramic layer composed mainly of ceramic particles by preventing loss of a ceramic coating layer by defining the density of a negative electrode active material layer.
Patent Document 1: Japanese Patent Application Laid-open No. H7-220759
Patent Document 2: Japanese Patent Application Laid-open No. 2005-327680
Patent Document 3: Japanese Patent Application Laid-open No. 2008-159333
The providing of an insulating layer composed mainly of an insulating inorganic material on the surface of an electrode active material layer (for example, the porous protective film described in Patent Document 1, to be referred to as a “protective layer”) contributes to preventing loss of the electrode active material, preventing internal short-circuiting and the like, and can be effective means for improving the reliability of a lithium secondary battery. However, when a protective layer containing a binder was formed on the surface of an electrode active material layer, since the binder in the protective layer penetrates into the electrode active material layer as charging and discharging are repeated, there was the risk of causing an increase in internal resistance or decrease in electrical conductivity and the like as compared with a lithium secondary battery of a configuration that does not have a protective layer.